This is another cliché I have trouble with. Mostly because I’ve been in academia and I know what types of books are out there. You will not find ancient tomes with descriptions of demons and a crucial piece of information that explains exactly how to resolve a supernatural situation.
For this one I blame the Buffy show (though I’m sure it wasn’t the first to abuse such an idea, merely the one that sticks in my mind). And why, why do these supernatural villains allow for the existence of these books in the first place. You would think that a demon who gets banished to hell would make a note the first time it happens and make a to-do list. “First thing I do when I get out in five thousand years is find that fricking book and burn it.”
Or maybe after getting banished the last time, spread the word? “Hey, Mort, yeah, I just got banished back here. Listen, if you get out, make sure the first thing you do is to destroy this book the Slayer has in her library. Yeah, do that and they won’t be able to send you back. Pass it on.”